class Solution {
public:
    bool vis[7][7];
    int n, m;
    bool exist(vector<vector<char>>& board, string word) {
        m = board.size(), n = board[0].size();
        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++) {
                if (board[i][j] == word[0]) {
                    vis[i][j] = true;
                    if (dfs(board, i, j, word, 1))
                        return true;
                    vis[i][j] = false;
                }
            }
        return false;
    }
    int dx[4] = {0, 0, -1, 1};
    int dy[4] = {1, -1, 0, 0};
    bool dfs(vector<vector<char>>& board, int i, int j, string& word, int pos) {
        if (pos == word.size()) {
            return true;
        }
        for (int k = 0; k < 4; k++) {
            int x = i + dx[k], y = j + dy[k];
            if (x >= 0 && x < m && y >= 0 && y < n && !vis[x][y] &&board[x][y] == word[pos])  //注意顺序
            {
                vis[x][y] = true;
                if (dfs(board, x, y, word, pos + 1))
                    return true;
                vis[x][y] = false;
            }
        }
        return false;
    }
};